In the state of the art, test and measurement devices are known that can be used for analyzing radio frequency signals. For instance, such a test and measurement device is a signal analyzer that usually comprises at least one analog to digital converter (ADC). Typically, the choice of the analog to digital converter used in the test and measurement device is a trade off between bandwidth and dynamics since a single analog to digital converter offers either a large number of bits at lower sample frequencies or a low number of bits at higher sample frequencies.
In the state of the art, two different methods are known to overcome the above mentioned issue wherein the bandwidth is enlarged by combining multiple analog to digital converters. These methods are called time-interleaving and frequency-interleaving methods. Both methods use multiple analog to digital converters in parallel wherein each analog to digital converter has a lower sample rate than required to sample the input signal.
In the time-interleaving method, the analog to digital converters sample the signal at different time instances such that the outputs of the converters are time-multiplexed in the digital domain to reconstruct the original signal. However, the main disadvantage of this method is the sensitivity to jitter. Thus, imaged spurs may occur in the recombined spectrum. In addition, the spurious free dynamic range (SFDR) is decreased.
On the other hand, the frequency-interleaving method uses an analog filter bank to divide the radio frequency signal into several sub-bands wherein each sub-band can be sampled at a lower sample rate. Then, the several sub-bands are forwarded to analog to digital converters in order to process these sub-bands digitally. In the digital domain, the different converter outputs are filtered, shifted back in frequency to their original position and added to reconstruct the original spectrum. Typically analog filters, for instance analysis filters, are used that are configured to cut a specific Nyquist band for a subsequent sub-sampling analog to digital converter. The original span is reconstructed by upsampling the corresponding band that has been filtered out by the filters previously, filtering the specific Nyquist band and adding the several subbands. However, the frequency responses of the filters are not known exactly and may change over time. In addition, delay and gain differences between the different sub-bands have to be considered by the synthesis filters appropriately. Accordingly, the reconstructed output signal may vary with regard to the original spectrum. Further, the frequency-interleaving method without mixers cannot be applied for radio frequency spans having a high center frequency.
Accordingly, there is a need for a test and measurement device as well as a method that are easy to use and that are able to provide improved characteristics with regard to the spurious free dynamic range and the output characteristics, in particular for signals having a high bandwidth.